Microelectromechanical systems (MEMS) typically involve at least one movable element in a self-contained package. For example, spatial light modulators such as interferometric or deflectable, reflective modulators may be contained in a package having a clear window offset from a substrate by a ring or other packaging support. MEMS-based switches may be encased in packages having protective caps offset from a substrate by similar components. Generally, MEMS devices will lie between the substrate and an offset cap or cover, as the devices need room to move when activated by an actuation signal.
This packaging type results in an operating environment for the MEMS devices that needs to be controlled. In many instances, the solution to controlling the environment inside the package involves establishing the environment inside the package and then hermetically sealing it to ensure that the environment remains constant. An example of such a hermetic process is discussed in U.S. Pat. No. 6,589,625, issued Jul. 8, 2003. The process of hermetically sealing packages contributes to the cost of the devices, as well as the complexity of the manufacturing process.
Another accommodation to the package environment is the use of getters. A getter is a material that is inserted into the package environment to adsorb moisture from the devices. A getter may adsorb water as it forms from high temperature operation, or may adsorb other materials that outgas from the operation of the device, as examples.
Control of the humidity of the interior of the package may have dramatic effects on the operation of the devices inside. High humidity may cause problems with stiction, a combination of sticking and friction, while completely dry environments may have other effects.
Other aspects of the environment may cause problems or result in increased performance. For example, reducing the partial pressure inside the package may speed the devices, as any damping effects of air between the movable element and the substrate may be reduced. This may result in increased performance, as the elements may move more quickly than in environments without reduced partial pressure.